Maximum Cable Length Calculation - Siemens 7MF4033 series User Manual

Installation

4.3.6.2 Maximum Cable Length Calculation

The maximum permissible single-pair cable length is 10,000 feet (3000 meters) or less as determined by
the following formula:
L = (65,000,000 / R x C) – (C
Formula Definitions:
L: The maximum total length of cable permitted to construct the network. L = feet when C is in pF/ft;
L = meters when C is in pF/meter.
R: The Network Resistance which is the ohmic sum of the current sense resistance and barrier
resistance (both return and supply), if any, in the network and the resistance of the wire.
C: Cable capacitance per unit length between one conductor and the other conductor connected to the
shield. C may be in pF/ft or pF/meter.
: Total input terminal capacitance of field instruments; the Primary Master is excluded. C
C
f
by the following formula:
C = (sum of all C
f
Where C is an integer (e.g., 1, 2, 3) corresponding to the input terminal capacitance of a Field
n
Instrument. C values are determined as follows:
n
FIELD INSTRUMENT CAPACITANCE
Less than 5000 pF
5000 pF to less than 10000 pF
10000 pF to less than 15000 pF
15000 pF to less than 20000 pF
20000 pF to less than 25000 pF
For field instruments without C
For an intrinsically safe installation, refer to the Control Drawing(s) in Appendix
B for entity parameters, which include C
installation data.
4-18
+ 10,000 / C)
f
values) x (5000)
n
values, use C
n
WARNING
i
C VALUE
n
1
2
3
4
5
= 1
n
transmitter input capacitance, and other
UMSITRPDS3-1
is given
f
November 2005